Because once they're accepted by the company, they're no longer yours, but the company's. Even if they say no to you, they may later fire you and claim your idea as theirs.
A difference must be made. On one side, there's your job and what you're paid to do for the company, how you use the company's time and resources, in order to contribute to their projects and achieve their goals.
On the other side, you, your own ideas, projects and goals, which hopefully shouldn't overlap those of the company; what you do in your own time with your own resources, at home, away from the office.
Reread your contract, and pay attention to any and all comments about Intellectual Property. Your contract most likely has a Non-Compete clause (unless your jurisdiction prohibits it, as it's the case in California).
If you're still okay with that, then think about the costs, risks and consequences of your idea. Many things that looked great on paper become a nightmare to implement, and eventually became lost at the bottom of the priority pool, as company resources become assigned to more urgent priorities.
Talk with as many people as possible about your idea. Let it become discussed, analyzed and criticized. Your direct boss may not be happy with it, but if your idea is good you may somehow become a John Houbolt and find a Robert Seamans who can bring attention from the higher-ups to your idea.